Food-Grade Microwave-Assisted Depolymerization of Grape Seed Condensed Tannins: Optimizing the Reaction Using Gallic Acid as a Nucleophile.

Abstract

Food waste has a significant social impact but can be revalued as a source of bioactive compounds, such as condensed tannins. This abundant biomass, corresponding to a polymeric antioxidant, must be depolymerized to become bioavailable. Previous studies have investigated polymer degradation into oligomers using high temperatures and expensive nucleophiles, often under conditions unsuitable for food applications. In the present investigation, it is proposed that the depolymerization of condensed tannins can occur under food-grade conditions using a Generally Recognized as Safe (GRAS) solvent by optimizing the reaction's heating method with microwave assistance and using gallic acid as a nucleophile. Thermal studies indicate that the degradation of total polyphenols content follows first-order kinetics and occurs above 80 °C in microwave. Depolymerization follows second-order kinetics, yielding epicatechin as the primary product with zero-order formation kinetics. The optimized factors were 80% v/v ethanol, 10 mg/mL polymeric tannins, and 5.88 mg/mL gallic acid. Under these conditions, the reaction efficiency was 99.9%, the mean particle diameter was 5.7 nm, the total polyphenols content was 297.3 ± 15.9 EAG mg/g, and the inhibition of ABTS●+ and DPPH● radicals was 93.5 ± 0.9% and 88.2 ± 1.5%, respectively. These results are promising for future scaling processes.